One of the main aspects to take into account in the development of an electronic device, especially mobile devices, is the area occupation. In the field of mobile devices, such as mobile phones, the reduction of the area occupation on the Printed Circuit Board (PCB) is a key point in order to create phones with much more functionalities without altering their dimensions. The focus, during the years, has been to integrate inside a chip, where possible, all those passive components such as resistors, capacitors and inductors which represent the main limit for the area reduction. Inside old generation phones, such components were SMD (Surface Mount Devices) mounted directly on the main board. Later on, thanks to improvement in the technology, these devices were placed inside the chip package, a methodology known as PDI (Passive Device Integration), and in some cases directly integrated inside the chip. However, when it comes to the microphone preamplifying path the implementation of this approach has not been possible due to the huge capacitance value of the decoupling capacitors needed between the microphone and the preamplifier.
FIGS. 1 and 2 show two known ways, single-ended and differential, respectively, to bias and connect the microphone circuit MCS, MCD to a preamplifier PAS, PAD using an RC network. The microphone circuit MCS, MCD comprises a microphone 3 and a biasing circuit RMB1, RMB2, RMB3, C1, C2, feed by a bias voltage VBIAS.
The DC bias voltage of the signal coming from the microphone circuit MCS, MCD at the output nodes MO, MO′ of the microphone circuit MCS, MCD will depend exclusively by the biasing circuit RMB1, RMB2, RMB3, C1, C2 and is usually different from the DC bias input voltage of the preamplifier PAS, PAD. The level shifting between the microphone 3 and the preamplifier PAS, PAD DC biasing voltages is commonly obtained using a decoupling capacitor CDEC that produces, with the preamplifier PAS, PAD input resistance, a first order high-pass filter whose corner frequency is generally lower than 20 Hz in order to avoid in-band audio signal perturbation.
More detailed representations of the differential preamplifier PAD are shown in FIG. 3 (inverting configuration) and FIG. 4 (non-inverting configuration).
In the inverting case, due to noise generation, input resistors R1A and R1B cannot have high resistance values (typically from 10 kOhm to 50 kOhm), whereas in the non-inverting solution resistors R3A and R3B are used only to bias the amplifiers OA inputs at a voltage VCM midway between ground and the supply voltage. Accordingly, resistors R3A and R3B don't contribute in noise generation and can be made with larger resistance values with respect to the inverting case (however, not more than some hundred of kOhms due to area occupation). In both cases, decoupling capacitors CDEC of more than 100 nF are needed and such large capacitance values would be difficult to integrate in a chip. In fact, with actual technologies on chip integration of a capacitor having such large capacitance value would require an area greater than 20 mm2 and this fact made the integrating approach practically unusable. US 2002/0125949 discloses the above problem of the waste of area due to the integration in the chip of the decoupling capacitor CDEC, confirming that the integration of the decoupling capacitors CDEC is practicable only for relatively reduced capacitance values. Moreover, unfortunately, even with the PDI methodology the decoupling capacitors CDEC can't be realized because of their high capacitance value and the fact that none of their terminals are connected to a fixed potential. This is the reason why all the existing known solutions use SMD capacitors. Since a preamplifier usually has several inputs (voice microphone, mono and stereo audio microphone, mono and stereo line-in, etc.) and each one could be differential, it is clear that on a mobile phone's PCB there are many SMD decoupling capacitors CDEC.
This is obviously a bottle neck for the area reduction strategy, and there is a strong felt need of trying to find a solution to this problem, till now without success. The same above described problem holds for other consumer devices different from mobile phones, such as portable MP3 players, digital photo cameras, digital audio recorders, video cameras, and in general in devices with audio communication and/or recording capabilities.
Resistance multipliers are already known in the state of art for example from the publication “Mini Sixties Plus”, Joseph Kreutz, ELECTOR, vol. 7/8, page 85 and from U.S. Pat. No. 5,652,537. However in the state of art, it was never proposed or suggested to adopt these multipliers in interface circuits for connecting a microphone circuit to a preamplifier with the aim of making possible the integration of decoupling capacitors CDEC. This is likely due to the fact that it is difficult to find a solution that adopts resistance multipliers such as the ones disclosed in the two above cited documents and which in the meantime is also adapted to provide a correct biasing, namely a fixed and stable biasing, of the preamplifier's input.